Please note that the timezone_identifiers_list function is not available in the most recent versions of PHP available for CentOS/RHEL as of this writing ( 5.1.6-23.2.el5_3 ).

I think the function is labeled as >= 5.1.0 in this documentation because it's possible to get the Date::Time class installed in 5.1 if you're compiling from scratch (or it's a documentation error).

I just spent a bunch of time trying to get it to work in 5.1 (installing timezonedb manually because the default PECL runs out of memory, etc.), but have decided to use a manually generated list instead. The actual timezone support seems to work fine in 5.1.